In order to reduce an image to digital form for manipulation by a computer, or in order for a computer to generate an image for display, it is customary to divide the image into a large number of picture elements (pixels) each of which can be described by a single colour value, or brightness value in monochrome. When the image is displayed, these pixels will be visible unless they can be made sufficiently small to be unresolved by the eye of a viewer, or are otherwise lost as a result of degradation of the image in subsequent optical processing or in the display system.
In the case of an image which is generated by a computer, sharp pixel boundaries are particularly visible when the intended image contains an abrupt edge which does not align with pixel boundaries, causing, in the case of rectangularly arranged pixels, jagged "staircase" artefacts. This effect is commonly referred to as "aliassing".
Even if the pixels are too small to be seen individually, there can be other consequences of the pixellation process. In particular, if the pixels are regularly distributed in space (e.g. as a rectangular grid), they can give rise to sampling anomalies. This problem arises when the image to be reproduced contains fine detail with a spatial frequency which exceeds the Nyquist frequency determined by the spacing of the pixels (i.e. a frequency of one half the reciprocal of the pixel spacing). In this case, as is well known from sampling theory, the spatial frequency components which exceed the Nyquist frequency are manifested at lower spatial frequencies. The visual effect of this form of "aliassing" is the introduction of "bands" and other clearly visible artefacts.
To minimise the above effects, it is clearly desirable to make the pixels as small as possible. However, the computation time required to process or to generate an image is proportional to the number of pixels, which should for this reason be minimised. In current image reproduction systems, a compromise is usually reached.
The term "image reproduction" is used herein to denote not only the reproduction on a display of an existing image, but also the reproduction of an image which is the result of computer manipulations, for example an image input to a computer by data defining coordinates of an image, an image synthesised within a computer, e.g. forms and patterns generated mathematically, or an image resulting from processing pixel data derived from an existing image.
In computer manipulation of moving images, it is current practice to maintain a fixed subdivision of the image area into pixels, which appear in the same place on consecutive frames. The eye therefore perceives the pixel structure in the moving image, because it is reinforced from one frame to the next. Aliassing artefacts are similarly reproduced in the same place on successive images, and are prominently perceived.
It is desirable to provide a system in which the effects of aliassing can be reduced by providing a recorded image with enhanced resolution.